The present invention relates to a double-layered ceramic heater consisting of an electrically insulating ceramic base plate and two patterned electric resistance heater element layers formed on both surfaces of the ceramic base plate or, more particularly, to a double-layered ceramic heater of the above described structure on which greatly improved uniformity can be obtained in the distribution of temperature.
In the manufacturing process of various kinds of electronic devices such as ICs, LSIs and the like which are prepared from a semiconductor silicon wafer by finely working thereon, the silicon wafers as the substrate are sometimes heated at a high temperature in various processes such as molecular beam epitaxy, chemical vapor-phase deposition method, sputtering method, ion-implantation method and the like by mounting the silicon wafer on a plate-formed heater in a vacuum chamber for the treatment.
A typical plate-formed heater used in the above mentioned purpose is a so-called ceramic heater having, preferably, a structure consisting of a base plate made from an electrically insulating ceramic material such as pyrolytic boron nitride, hereinafter referred to as PBN, and a resistance heater element layer formed thereon from pyrolytic graphite, hereinafter referred to as PG, as is disclosed in Japanese Patent Kokai 63-241921. Ceramic heaters of this type are advantageous as compared with conventional tantalum-wire heaters in respect of the handiness of easy mounting and absence of troubles due to thermal deformation, line break and short-circuiting in addition to the relatively good uniformity of the temperature distribution as an inherency in plate heaters in general.
In order to ensure further improved uniformity of temperature distribution over the whole surface of such a ceramic heater, a proposal is made in Japanese Utility Model Kokai 3-30393 to increase the quantity of heat evolution per unit area with a gradient in the radial direction from the center to the periphery of a circular ceramic heater. It is also disclosed in Japanese Patent Kokai 4-112478 to form two resistance heater element layers on both surfaces of a ceramic base plate with an object to improve the temperature balance in the direction of thickness of the ceramic base plate. The improvement obtained with these double-layered ceramic heaters is still insufficient in respect of the uniformity of temperature distribution because the resistance heater element layer is always patterned on the surface of the ceramic base plate to have a gap area where no heat can be evolved between the patterned heat-evolving or heating areas.